In the fabrication of semiconductor devices from a silicon wafer, a variety of semiconductor processing equipment and tools are utilized. One of these processing tools is used for polishing thin, flat semiconductor wafers to obtain a planarized surface. A planarized surface is highly desirable on a shadow trench isolation (STI) layer, inter-layer dielectric (ILD) or on an inter-metal dielectric (IMD) layer, which are frequently used in memory devices. The planarization process is important since it enables the subsequent use of a high-resolution lithographic process to fabricate the next-level circuit. The accuracy of a high resolution lithographic process can be achieved only when the process is carried out on a substantially flat surface. The planarization process is therefore an important processing step in the fabrication of semiconductor devices.
A global planarization process can be carried out by a technique known as chemical mechanical polishing, or CMP. The process has been widely used on ILD or IMD layers in fabricating modern semiconductor devices. A CMP process is performed by using a rotating platen in combination with a pneumatically-actuated polishing head. The process is used primarily for polishing the front surface or the device surface of a semiconductor wafer for achieving planarization and for preparation of the next level processing. A wafer is frequently planarized one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer can be polished in a CMP apparatus by being placed on a carrier and pressed face down on a polishing pad covered with a slurry of colloidal silica or aluminum.
A polishing pad used on a rotating platen is typically constructed in two layers overlying a platen, with a resilient layer as an outer layer of the pad. The layers are typically made of a polymeric material such as polyurethane and may include a filler for controlling the dimensional stability of the layers. A polishing pad is typically made several times the diameter of a wafer in a conventional rotary CMP, while the wafer is kept off-center on the pad in order to prevent polishing of a non-planar surface onto the wafer. The wafer itself is also rotated during the polishing process to prevent polishing of a tapered profile onto the wafer surface. The axis of rotation of the wafer and the axis of rotation of the pad are deliberately not collinear; however, the two axes must be parallel. It is known that uniformity in wafer polishing by a CMP process is a function of pressure, velocity and concentration of the slurry used.
A CMP process is frequently used in the planarization of an ILD or IMD layer on a semiconductor device. Such layers are typically formed of a dielectric material. A most popular dielectric material for such usage is silicon oxide. In a process for polishing a dielectric layer, the goal is to remove typography and yet maintain good uniformity across the entire wafer. The amount of the dielectric material removed is normally between about 5000 A and about 10,000 A. The uniformity requirement for ILD or IMD polishing is very stringent since non-uniform dielectric films lead to poor lithography and resulting window-etching or plug-formation difficulties. The CMP process has also been applied to polishing metals, for instance, in tungsten plug formation and in embedded structures. A metal polishing process involves a polishing chemistry that is significantly different than that required for oxide polishing.
Important components used in CMP processes include an automated rotating polishing platen and a wafer holder, which both exert a pressure on the wafer and rotate the wafer independently of the platen. The polishing or removal of surface layers is accomplished by a polishing slurry consisting mainly of colloidal silica suspended in deionized water or KOH solution. The slurry is frequently fed by an automatic slurry feeding system in order to ensure uniform wetting of the polishing pad and proper delivery and recovery of the slurry. For a high-volume wafer fabrication process, automated wafer loading/unloading and a cassette handler are also included in a CMP apparatus.
As the name implies, a CMP process executes a microscopic action of polishing by both chemical and mechanical means. While the exact mechanism for material removal of an oxide layer is not known, it is hypothesized that the surface layer of silicon oxide is removed by a series of chemical reactions which involve the formation of hydrogen bonds with the oxide surface of both the wafer and the slurry particles in a hydrogenation reaction; the formation of hydrogen bonds between the wafer and the slurry; the formation of molecular bonds between the wafer and the slurry; and finally, the breaking of the oxide bond with the wafer or the slurry surface when the slurry particle moves away from the wafer surface. It is generally recognized that the CMP process is not a mechanical abrasion process of slurry against a wafer surface.
While the CMP process provides a number of advantages over the traditional mechanical abrasion type polishing process, a serious drawback for the CMP process is the difficulty in controlling polishing rates at different locations on a wafer surface. Since the polishing rate applied to a wafer surface is generally proportional to the relative rotational velocity of the polishing pad, the polishing rate at a specific point on the wafer surface depends on the distance from the axis of rotation. In other words, the polishing rate obtained at the edge portion of the wafer that is closest to the rotational axis of the polishing pad is less than the polishing rate obtained at the opposite edge of the wafer. Even though this is compensated for by rotating the wafer surface during the polishing process such that a uniform average polishing rate can be obtained, the wafer surface, in general, is exposed to a variable polishing rate during the CMP process.
Recently, a chemical mechanical polishing method has been developed in which the polishing pad is not moved in a rotational manner but instead, in a linear manner. It is therefore named as a linear chemical mechanical polishing process, in which a polishing pad is moved in a linear manner in relation to a rotating wafer surface. The linear polishing method affords a more uniform polishing rate across a wafer surface throughout a planarization process for the removal of a film layer from the surface of a wafer. One added advantage of the linear CMP system is the simpler construction of the apparatus, and this not only reduces the cost of the apparatus but also reduces the floor space required in a clean room environment.
After they are subjected to the CMP process, wafers are typically cleaned using a wafer scrubbing and drying system, such as a Dai Nippon Screen (DNS) model AS-2000 system, for example. Such a post-CMP wafer cleaning and drying system and method includes three main stages: scrubbing of both sides of the wafer in a double-sided brush-scrubbing chamber (DBC), scrubbing of the upper surface of the wafer in a top brush scrubbing chamber (TBC), and finally, drying of the scrubbed wafer in a dry task chamber (DTC). The latter two steps may be carried out in the same chamber. These scrubbing and drying steps remove particulate contaminants as well as residual slurry from the wafer after the CMP operation.
A schematic of a typical wafer-scrubbing assembly 10 for a double-sided brush-scrubbing chamber (DBC) is shown in FIG. 1A. The assembly 10 includes a pair of roller mounts 12, each of which supports typically three rotating chuck rollers 14. As shown in FIG. 1B, each of the chuck rollers 14 typically includes a wide base 32, a narrower middle section 33 and a relatively thin top flange 34 that defines a wafer notch 35 in the top of the middle section 33. The roller mounts 12 are movable toward and away from each other. At least one of the roller mounts 12 is provided with a chuck sensor 16 fitted with a light sensor 17 that is disposed adjacent to a light source 19 which emits a light beam 18. A rotatable bottom brush holder 22 that mounts a bottom scrub brush 24 is provided between the roller mounts 12, and a top brush holder 26, to which is mounted a top scrub brush 28, is disposed above the bottom brush holder 22.
In operation, the chuck rollers 14 move toward each other to “catch” the wafer 30, with the edges of the wafer 30 inserted in the wafer notches 35 of the respective chuck rollers 14 as shown in FIG. 1B. The chuck rollers 14 rotate in the same direction to rotate the wafer 30 there between. Simultaneously, the bottom scrub brush 24 is rotated against the bottom surface of the wafer 30 and the top scrub brush 28 is rotated against the top surface of the wafer 30 to remove particulate contaminants and residual slurry adhering to these wafer surfaces.
As shown in FIG. 1B, when the wafer 30 is properly engaged by each of the multiple chuck rollers 14, the edge of the wafer 30 is inserted in the wafer notch 35, between the top flange 34 and the middle section 33 of each chuck roller 14. Accordingly, when the chuck rollers 14 are disposed in the proper wafer-engaging position, the light beam 18 emitted by the light source 19 is aligned with and received by the light sensor 17 of the chuck sensor 16, as shown in FIG. 1A. This indicates to the control system for the assembly 10 that the wafer 30 is properly engaged for scrubbing. However, as shown in FIG. 1C, one edge of the wafer 30 may be inadvertently missed by the wafer notch 35 of one or more of the chuck rollers 14, such that the wafer 30 is supported on the top surface of the relatively thin top flange 34 of the chuck roller 14. In this configuration, the light beam 18 is still aligned with and intercepted by the light sensor 17 of the chuck sensor 16, which erroneously indicates to the assembly control system that the wafer 30 is properly engaged by the chuck rollers 14 for rotation and scrubbing. Consequently, damage or breakage of the wafer 30 may result upon subsequent contact of the bottom scrub brush 24 and/or top scrub brush 28 with the wafer 30. Accordingly, chuck rollers having a new and improved configuration for ensuring proper engagement of the chuck rollers with a wafer during a wafer scrubbing operation, are needed.
A perspective view of a typical conventional wafer holder assembly 40 for a top brush scrubbing chamber (TBC) and dry task chamber (DTC) of a wafer cleaning and drying system, particularly a DNS model AS-2000 system, is shown in FIG. 2A. The wafer holder assembly 40 includes a motor housing 42 which houses a motor (not shown). A chuck base 44 having multiple elongated chuck arms 46 is rotatably engaged by the motor in the motor housing 42. A chuck pin 48 is upward-standing from the extending end portion of each chuck arm 46.
As shown in FIG. 22, each chuck pin 48 includes an attachment flange 49 for attachment of the chuck pin 48 to the corresponding chuck arm 46. A pin body 50 extends upwardly from the attachment flange 49, and a wafer support surface 51, from which extends a wafer support pin 52, is provided in the pin body 50. The pin body 50 has a diameter of typically 10 mm. A wafer catch flange 53, having a flat wafer engaging surface 54, extends upwardly beyond the wafer support surface 51.
As shown in FIG. 2C, during scrubbing of the top surface of a wafer 30 and subsequent drying of the wafer 30, the wafer 30 is supported on the wafer support pin 52 of each of the corresponding chuck pins 48, with the flat wafer engaging surface 54 on the wafer catch flange 53 of each chuck pin 48 firmly engaging the outer edge of the wafer 30. As the chuck base 44 is rotated at speeds of typically 1500-2500 rpm, the wafer 30 is likewise rotated by the chuck pins 48. One of the problems which is frequently encountered during this spinning of the wafer 30 is that the wafer 30 becomes disengaged from one or more of the chuck pins 48 by sliding upwardly against the wafer engaging surface 54, thereby causing damage or breakage to the wafer 30 as well as potential damage to the chamber in which the process is carried out. Accordingly, chuck pins having a new and improved configuration for maintaining proper engagement of the chuck pins with a wafer during wafer scrubbing and wafer drying operations, are needed.
An object of the present invention is to provide new and improved chuck rollers for rotating a wafer in a wafer cleaning and drying system.
Another object of the present invention is to provide chuck rollers each having a novel configuration which ensures proper engagement of each chuck roller with a wafer for rotation of the wafer during a wafer scrubbing process.
Still another object of the present invention is to provide chuck rollers for a wafer scrubbing and drying system, which chuck rollers each have a roller head for ensuring proper engagement of a wafer with each chuck roller and preventing wafer damage during a wafer scrubbing process.
Yet another object of the present invention is to provide new and improved chuck pins for supporting a wafer in a wafer cleaning and drying system.
A still further object of the present invention is to provide new and improved chuck pins each having a novel configuration which maintains proper engagement of the chuck pins with a wafer during wafer scrubbing and drying processes.